In semiconductor manufacturing, an important problem is optimizing the recipes for Chemical Vapor Deposition (CVD) processes, such as BoroPhosphoSilicate Glass (BPSG) processes, in order to meet specific performance criteria. For example, in cell formation processing, manufacturers typically use a 3-step process consisting of (1) BPSG CVD, (2) plasma etch, and (3) wet clean/etch. FIG. 1 illustrates an exemplary prior art 3-step BPSG CVD process for cell formation processing, according to one embodiment of the present invention.
The purpose of this 3-step process is to etch out cylindrical cells on the substrate, which are then typically filled with conductive materials and used to manufacture computer components, such as memory. These cells need to have a minimum diameter along their depth and to be separated by a minimum distance, also along their depth, in order to maintain their desired conductive properties. At the same time, manufacturers want to etch these cells as close as possible to one another, in order to maximize their yield: the closer these cells are to each other, the more cells fit on a single wafer, and thus the more components can be manufactured out of a single wafer.
FIG. 2 illustrates an exemplary prior art diagram of the top view of a single cell 600 with an elliptical shape along the X-axis and Y-axis. Each semiconductor wafer contains many of these cells next to each other. The sidewall profiles of these cells are typically measured by cutting the wafer either along the X-axis 610 or along the Y-axis 620 and then using an imaging method such as Scanning Electron Microscopy to generate an image of the cross-section of the cells.
FIG. 3 illustrates a prior art diagram of ideal and typical sidewall profiles in cell formation processing. The ideal cell sidewall profile 210 for placing as many cells as possible on a single wafer is a perfect cylinder. Typical recipe design is based on trial-and-error optimization, and generates sidewall profiles 220 that typically have more than 1000 Å (Angstroms) total deviation from the completely vertical cylindrical profile as illustrated in FIG. 3.
The problem at hand, then, is a systematic method for optimizing the recipes for cell formation processing, so that the actual results are much closer to the ideal cylindrical profile than they are today.